Two analogous polyhedron-based MOFs with high Lewis basic sites

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Two analogous polyhedron-based MOFs with high Lewis basic sites and open metal sites: significant CO2 capture and gas selectivity performance Bing Liu, Shuo Yao, Xinyao Liu, Xu Li, Rajamani Krishna, Guanghua Li, Qisheng Huo, and Yunling Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b10795 • Publication Date (Web): 07 Sep 2017 Downloaded from http://pubs.acs.org on September 9, 2017

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ACS Applied Materials & Interfaces

Two analogous polyhedron-based MOFs with high Lewis basic sites and open metal sites: significant CO2 capture and gas selectivity performance Bing Liu,a Shuo Yao,a Xinyao Liu,a Xu Li,b Rajamani Krishna,c Guanghua Li,a Qisheng Huoa and Yunling Liu*a a

State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry,

Jilin University, Changchun 130012, P. R. China b

Department of Chemical and Bimolecular Engineering, National University of Singapore,

117576, Singapore. c

Van ‘t Hoff Institute for Molecular Sciences, University of Amsterdam, Science Park 904, 1098

XH Amsterdam, The Netherlands. KEYWORDS: polyhedron Metal-organic frameworks, MOP cages, porous materials, CO2 capture, gas separation.

ACS Paragon Plus Environment

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ABSTRACT

By means of modulating the axial ligand and adopting supermolecular building blocks (SBBs) strategy,

two

polyhedron-based

metal-organic

[Cu6(C17O9N2H8)3(C6H12N2)(H2O)2(DMF)2]·3DMF

8H2O

frameworks (JLU-Liu46)

(PMOFs) and

[Cu6(C17O9N2H8)3(C4H4N2)(H2O)2(DMF)2]·3DMF 8H2O (JLU-Liu47), which possess high density of Lewis basic sites (LBSs) and open metal sites (OMSs), have been successfully synthesized. For the reason that the size of axial ligand in JLU-Liu47 is smaller than that in JLU-Liu46, JLU-Liu47 shows larger pore volume and higher BET surface area. And then, the adsorption ability of JLU-Liu47 for some small gases is better than JLU-Liu46. It is worthwhile mentioning that both of the two compounds exhibit outstanding adsorption capability for CO2 ascribed to the introducing of urea groups. In addition, the theoretical ideal adsorbed solution theory (IAST) calculation and transient breakthrough simulation indicate that JLU-Liu46 and JLU-Liu47 should be potential materials for gas storage and separation, particularly for CO2/N2, CO2/CH4 and C3H8/CH4 separation.

INTRODUCTION The sharp increasing of CO2 in the atmosphere, which produced by the burning of fossil fuels (such as coal, oil and natural gas), has led to global warming.1-5 Therefore, reducing the CO2 emissions and developing clear energy source have become the most urgent environmental issues. Currently, promoting the CO2 capture and separation technologies will play a fatal role in dealing with the problems.


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Metal-organic frameworks (MOFs), which have significant applications in gas capture, storage and separation, are a new kind of promising advanced materials compared to the traditional porous zeolites and carbon materials.6-13 Besides, metal-organic polyhedral (MOP) which owns large voids cage and open-channel-type is a prominent building unit to construct MOFs with large pore volume, high surface area and excellent gas uptake capacity.14-17 For the sake of constructing polyhedron-based metal-organic frameworks (PMOFs), supermolecular building blocks (SBBs) approach is considered to be an effective strategy which has been used to configure a series of classic PMOFs, such as rht-MOFs,18 gea-MOFs19 and fcu-MOFs.20 In addition, it also indicates that the ligands, with functional groups such as amino, imino, amide, urea, imidazole and triazole in the PMOFs, may significantly improve the storage and separation performance for CO2. Therefore, many PMOFs modified with functional groups have been reported, such as NU-100,21 Cu-TPBTM,22 PCN-6x series,23 Cu-TDPAT,24 Cu-TATAB,25 HUNST-526 and so forth. As for PCN-61 and Cu-TPBTM, although the pore size, surface area and the number of open metal sites (OMSs) for the two MOFs are almost same, the CO2 adsorption performance is different because the functional groups in PCN-61 is -C≡C- and that is -NH-CO- in Cu-TPBTM. In terms of isophthalate moiety making it convenient to construct MOP cage, the tetracarboxylic acid ligand with two isophthalate moieties is commendable candidate to synthesize PMOFs. However, most tetracarboxylic ligands are conditioned to assemble nbo topology without MOP cages, such as MOF-505,27 MOF-101,28 UTSA-80,29 PCN-16,30 NOTT10531 and NU-135.32 In our preliminary work, a ligand with both tetracarboxylic and triazole moieties has been selected to successfully construct two novel PMOFs materials JLU-Liu20 and JLU-Liu21, which display significant capture capacity of CO2.33 In order to improve the


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properties of CO2 adsorption and separation, urea groups which possesses high Lewis basic sites (LBSs) have been introduced as functional group to replace the triazole moieties in the tetracarboxylic ligand and a less used ligand 5,5’-(carbonylbis(azanediyl))diisophthalic acid (H4L) is successfully prepared. Up to now, only two MOFs (NU-60134 and Eu4[L]335) based on H4L ligand have been reported and neither of them possess MOP cage nor adsorption abilities. Opportunely,

we

have

successfully

prepared

a

PMOF

[Cu6(C17O9N2H8)3(C6H12N2)(H2O)2(DMF)2]·3DMF 8H2O (JLU-Liu46) which is made up of three different kinds of MOP cages and possesses high density of OMSs and LBSs. In consideration of the 1,4-diazabicyclo[2.2.2]-octane (DABCO) in the framework occupies a certain amount of space and a similar but smaller pyrazine ligand is adopted to construct the same

framework.

Just

as

what

we

expected,

another

analogous

PMOF

[Cu6(C17O9N2H8)3(C4H4N2)(H2O)2(DMF)2]·3DMF 8H2O (JLU-Liu47) with larger pore volume than JLU-Liu46 is successfully synthesized. Both of the two compounds exhibit excellent properties in small gases (CO2, CH4, C2H6 and C3H8) adsorption, especially for CO2. Meanwhile, the simulation method of ideal adsorbed solution theory, breakthrough and Grand Canonical Monte Carlo Simulations are applied to evaluate the gas selective separation abilities. The simulation results indicate that both JLU-Liu46 and JLU-Liu47 are promising materials for gas capture and separation. EXPERIMENTAL SECTION Materials and Methods. The ligand of H4L was synthesized in accordance with the literature procedure.34 The structural characterization is shown in Scheme S1 and Fig. S1. 1H NMR (300 MHz, DMSO-d6): δ (ppm): 13.24 (s, 4H), 9.64 (d, 2H), 8.34 (d, J=1.5 Hz, 4H), 8.12 (d, J=3 Hz, 2H).


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All chemicals were purchased from chemical company, and used without additional purification. Powder X-ray diffraction (PXRD) pattern were recorded on a Rigaku D/max-2550 diffractometer with Cu-Kα radiation (λ = 1.5418 Å). Elemental analyses (C, H and N) were performed on a vario MICRO elementary analyzer. The thermal gravimetric analyses (TGA) were carried out on a TGA Q500 thermogravimetric analyzer with a heating rate of 10 °C min-1 in air condition. Synthesis of Compounds. Synthesis of JLU-Liu46: A mixture of Cu(BF4)2·6H2O (7.5 mg, 0.012 mmol), H4L (2 mg, 0.005 mmol), 1,4-diazabicyclo[2.2.2]-octane (DABCO) (0.1 mL, 2 g in 10 mL DMF), N, Ndimethylformamide (DMF) (1 mL), ethanol (0.5 mL), H2O (0.5 mL) and 0.35 mL HNO3 (2.7M in DMF) were put into a 20 mL vial, then the vial was transferred to an oven and heated at 85 oC for 12 hours. Blue crystals were gathered, washed with DMF and air-dried (58% yield based on Cu(BF4)2·6H2O). Elemental analysis (%) calculated for JLU-Liu46 C72H91Cu6N13O42: C, 39.45; H, 4.19; N, 8.31. Found: C, 39.93; H, 4.23; N, 8.41. The experimental and simulated PXRD patterns are identical, reveal that the as-synthesized compound is obtained as the pure phase (Fig. S9a). Synthesis of JLU-Liu47: A mixture of Cu(BF4)2·6H2O (7.5 mg, 0.012 mmol), H4L (3 mg, 0.0075 mmol), pyrazine (0.15 mL, 2 g in 10 mL DMF), DMF (1 mL), ethanol (0.75 mL), H2O (0.75 mL) and 0.45 mL HNO3 (2.7 M in DMF) were added to a 20 mL vial, then the vial was put into an oven and heated at 85 oC for 10 hours. Blue crystals were collected, washed with DMF and air-dried (54% yield based on Cu(BF4)2·6H2O). Elemental analysis (%) calculated for JLULiu47 C70H83Cu6N13O42: C, 38.93; H, 3.87; N, 8.43. Found: C, 38.98; H, 3.91; N, 8.38. The


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experimental and simulated PXRD patterns are identical revealed that the as-synthesized products are phase pure (Fig. S10a). Single Crystal X-ray Crystallography. Both the crystallographic data of JLU-Liu46 and JLU-Liu47 were recorded on a Bruker Apex II CCD diffractometer using graphitemonochromated Mo-Kα (λ = 0.71073 Å) radiation at 20 oC. The structures were solved by direct methods and refined by full-matrix least-squares on F2 using version 5.1.36 From the difference Fourier map, all non-hydrogen atoms were definitely found and refined anisotropically. The hydrogen atoms of the ligands were generated theoretically onto the specific atoms and refined isotropically with fixed thermal factors. As the guest molecules were highly disordered and could not be modeled properly, the diffused electron densities resulting from them were removed by the SQUEEZE routine in PLATON and the results were appended in the CIF file. The reported refinements are of the guest-free structures using the *.hkp files produced on a SQUEEZE routine operations. The final formula of JLU-Liu46 and JLU-Liu47 were derived from crystallographic data combined with elemental and thermogravimetric analysis data. Crystallographic data for JLU-Liu46 and JLU-Liu47 (1555805 and 1555806) have been deposited with Cambridge Crystallographic Data Centre. Data can be found free of charge upon request at www.ccdc.cam.ac.uk/data_request/cif. Crystal data and structure refinement is summarized in Table S1. The topology analysis for JLU-Liu46 and JLU-Liu47 was calculated by using TOPOS 4.0 software.37 Gas Adsorption Measurements. The adsorption measurements of N2, CO2, CH4, C2H6 and C3H8 were carried out on a Micromeritics ASAP 2020. In order to completely get rid of the guest solvent molecules, JLU-Liu46 and JLU-Liu47 were soaked in ethanol and replaced with absolute ethanol, 12 cycles for 3 days, and then activated at 95 °C for 10 h under the vacuum.


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The non-volatile solvent molecules have mostly been removed during the activation process as confirmed by the TGA data (Fig. S 11, 12). RESULTS AND DISCUSSION Single-crystal X-ray diffraction analysis reveals that JLU-Liu46 crystallizes in the tetragonal crystal system with P4/mnc space group. The framework is aggregated with typical copper paddlewheels and two types of organic linkers (Fig. S2), L4- as major linker and the DABCO as axial ligand, respectively (Fig. 1a). As illustrated in Fig. 1b, JLU-Liu46 has three types of cage with large Connolly surface area. The first kind of cage is the classical MOP-1 with cuboctahedron (cuo) geometry (M12L24), which consists of 12 copper paddlewheels and 24 L4ligands. The second kind of cage shows truncated tetrahedron (T-Td) configuration (M12L6) with 12 copper paddlewheels, 6 L4- ligands and 4 DABCO ligands. The third cage is the largest one with truncated octahedron geometry (T-Oh) (M24L16) which is constructed from 24 copper paddlewheels, 16 L4- and 8 DABCO. The internal diameters of the three kinds of cages are 31 (T-Oh), 12 (cuo) and 10 (T-Td) Å (regardless of the van der Waals radii), respectively (Fig. 1c and S5). Then, the assembling of the three different kinds of polyhedron cages leads to the formation of a 3D PMOF. Moreover, JLU-Liu46 possess multiple pore systems, the diameter of the biggest square channel is approximate to be 6.5 Å × 6.5 Å along the a axis (regardless of the van der Waals radii) (Fig. S3, 4). Such cages provide a beneficial environment for better access of natural gas and the consequent host-guest interaction. From the view of the topology, the Cupaddlewheels building unit can be simplified as a square and L4- is seen as a pair of triangle geometries. The overall framework forms a (3, 4)-connected network, and the Schläfli symbol is {62.82.92}2{62.8}4{62.9}2{63.8.102}, which is same to our previously reported JLU-Liu20 and


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JLU-Liu21 materials (Fig. S8). The total accessible volume of JLU-Liu46 calculated by PLATON is 63.0%.

Fig. 1 Single-crystal structure for JLU-Liu46 and JLU-Liu47: Cu paddlewheel, organic L4ligand and the axial ligand DABCO (a) and pyrazine (d); three types of cage with Connolly surface areas for JLU-Liu46 (b) and JLU-Liu47 (e); (c) arrangement of the three kinds of cage for JLU-Liu46 and JLU-Liu47. As an axial ligand, DABCO unit connects two adjacent paddlewheels and doubly supports the framework, which greatly enhances the framework rigidity and stability. However, it reduces the size of window for the framework at the same time. In order to mitigate the disadvantage,


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pyrazine, which has the same shape but smaller size to DABCO, is selected to synthesize another analogous PMOF. Fortunately, the presumptive PMOF JLU-Liu47 is successfully obtained, which possesses the similar framework to JLU-Liu46 (Fig. 1c-e and S6). The structure difference between them is the axial ligand in JLU-Liu46 is DABCO, while, that is pyrazine in JLU-Liu47. The DABCO with an axial length of 2.6 Å is connected to two paddlewheels with a distance of 6.9 Å in JLU-Liu46. By contrast, the pyrazine with an axial length of 2.8 Å is linked to two paddlewheels with a distance of 7.1 Å in JLU-Liu47. Hence, the windows in JLU-Liu46 are smaller than those in JLU-Liu47 (Fig. S7). Moreover, the larger transverse dimension of DABCO (4.0 Å) makes JLU-Liu46 own smaller accessible pore volumes (63.0%) than JLULiu47 (66.5%), which can be verified by PLATON. Therefore, the adsorption performance of JLU-Liu47 will be changed compared to JLU-Liu46. Thermal Stability Analyses The thermal stability of JLU-Liu46 and JLU-Liu47 are estimated by variable-temperature powder X-ray diffraction (VTPXRD) and thermogravimetric analysis (TGA), cooperatively. In the respect of VTPXRD, both of the compounds can keep its integrity until 200 oC (Fig. S9b, 10b). From the TGA point of view, JLU-Liu46 can be steady to 250 oC, and the 42.9% weight loss is attributed to the release of guest molecules. Subsequently, the framework collapsed and displays 37.2% (calcd. 36.8%) weight loss (Fig. S11). Similarly, JLU-Liu47 shows 32.3% weight loss before 250 oC and further 38.9% (calcd. 37.8%) weight loss until 450 oC (Fig. S12). Gas Adsorption of JLU-Liu46 and JLU-Liu47 The reversible type I isotherm obtained by N2 adsorption measurement at 77 K verify the permanent porosity for JLU-Liu46 and JLU-Liu47 (Fig. S13). The Brunauer-Emmett-Teller


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(BET) surface area of JLU-Liu46 is calculated to be 1787 m2 g-1, and the Langmuir surface area is 2400 m2 g-1. The experimental total pore volume resulted from the N2 isotherm was found to be 0.87 cm3 g-1, which is pretty close to the theoretical pore volume (0.91 cm3 g-1). Owing to the multiple pore systems, high density of OMSs/LBSs and surface areas of JLU-Liu46, it also features good adsorption capacity for other small gases. The CO2 uptake of JLU-Liu46 is explored to be 185 (36.3 wt%) and 104 (20.4 wt%) cm3 g-1 at 273 and 298 K under 1 atm, respectively, which are much higher than most reported MOFs materials, such as Zn-MOF-74 (17.6 wt%),38 Cu3(BTC)2 (18.4 wt%)39 and PCN-6 (15.9 wt%).40 Due to the high density of LBSs and OMSs which provide a larger number of force sites for CO2 molecules, JLU-Liu46 exhibits outstanding adsorption capacity of CO2. This result suggests that JLU-Liu46 could be a well candidate for CO2 capture and storage. At zero loading, the adsorption enthalpy of JLU-Liu46 is 32 kJ mol-1 (Fig. S15a), which higher than the MOFs based on Cu-paddlewheel unit and multidentate carboxylic acid ligand, such as PCN-124 (26.3),41 PCN-66 (26.2),23 PCN-61 (22.0),23 NOTT-125 (25.4)42 and NOTT-122 (24.5).43 The adsorption of some other light hydrocarbons CH4, C2H6 and C3H8 are also measured at 273 and 298 K under 1 atm, respectively. The maximum uptake for CH4 is 31 and 17 cm3 g-1, C2H6 is 158 and 107 cm3 g-1 and C3H8 is 171 and 154 cm3 g-1, respectively (Fig. 2b-d). Furthermore, the Qst of CH4, C2H6 and C3H8 is 15, 28 and 33 kJ mol-1, respectively (Fig. S15b-d).


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Fig. 2 (a) CO2; (b) CH4; (c) C2H6; (d) C3H8 gas absorption-desorption isotherms for JLU-Liu46 at 273 and 298 K under 1 atm. As far as JLU-Liu47, its BET surface areas and Langmuir surface area were calculated to be 1800 and 2493 m2 g-1, respectively (Table S2). Rationally, the CO2, CH4, C2H6 and C3H8 adsorption performance for JLU-Liu47 is better than JLU-Liu46. At 273 and 298 K, the maximum adsorption of CO2 is 192 (37.7 wt%) and 108 (21.2 wt%) cm3 g-1 (Fig. 3a). Moreover, the CO2 adsorption capacity of JLU-Liu47 raise to 4.3 wt% and JLU-Liu46 is 2.9 wt% at 298 K and 0.15 bar, which is pointedly higher than many reported Cu-MOFs under the same condition (Table S5). Significantly, in comparison with triazole functionalized MOFs, JLU-Liu20, JLULiu47 shows higher CO2 uptake at 1 bar (Table S3, S4). The phenomenon is due to the size of pyrazine is smaller than DABCO and JLU-Liu47 has larger pore volume than JLU-Liu46. It is notable that the enthalpy of CO2 for JLU-Liu47 reaches up to 35 kJ mol-1 (Fig. S16a), which is due to the high OMSs density, anionic skeletons, multiple pore systems and higher surface area of JLU-Liu47 frameworks coordinately provide stronger interactions to the CO2 molecules.


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Similarly, JLU-Liu47 has better adsorption capacity to small gas molecules than JLU-Liu46. The maximum adsorption of CH4 is 36 and 21 cm3 g-1, C2H6 is 191 and 125 cm3 g-1 and C3H8 is 217 and 182 cm3 g-1, respectively (Fig. 3b-d). Furthermore, the Qst of CH4, C2H6 and C3H8 is 16, 28 and 34 kJ mol-1, respectively (Fig. S16b-d).

Fig. 3 (a) CO2; (b) CH4; (c) C2H6; (d) C3H8 gas absorption-desorption isotherms for JLU-Liu47 at 273 and 298 K under 1 atm. Separation Behaviors of JLU-Liu46 and JLU-Liu47 For the purpose of industrial applications, the selective separation behavior of JLU-Liu46 and JLU-Liu47 for CO2/CH4, C2H6/CH4 and C3H8/CH4 were calculated by IAST. Subsequently, the fitting parameters are used for the prediction of multi-component adsorption by IAST. The selectivity of CO2/CH4 (50% and 50%, 5% and 95%) is 9.8 and 6.5 for JLU-Liu46 at 298 K and 1 atm (Fig. 4a, b), which are comparable to ZIF-844 and some carbon materials45 under the same


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conditions. The selectivity of C2H6 over CH4 (50% and 50%), C3H8 over CH4 (50% and 50%) is 17 and 169 at 298 K and 1 atm, respectively (Fig. 4c, d).

Fig. 4 CO2, CH4, C2H6 and C3H8 adsorption isotherms at 298 K along with the dual-site Langmuir Freundlich (DSLF) fits (a, c); Gas mixture adsorption selectivity are predicted by IAST at 298 K and 100 kPa for JLU-Liu46 (b, d). Besides, with respect to JLU-Liu47, the selectivity of CO2 over CH4 (50% and 50%, 5% and 95%) is 10.4 and 7.3 (Fig. 5a, b). As shown in Fig. 5, the selectivity for equimolar binary mixtures of C2H6 and CH4, C3H8 and CH4 is 17 and 168 (Fig. 5c-d), respectively, which are similar to JLU-Liu46. The selectivity of C3H8 over CH4 for JLU-Liu46 and JLU-Liu47 is comparable to some reported MOFs including LIFM-26 (50),46 FJI-C1 (78.7),47 UTSA-35a (80),48 FIR-7aht (80),49 eea-MOF-4 (136)50 and eea-MOF-5 (156).50 Considering the difference of polarizability and quadrupole moment between CO2 and CH4, the significant separation for CH4 of JLU-Liu46 and JLU-Liu47 can be mainly attributed to their multiple cage features, in which, the high density of OMSs and LBSs provide more interaction force between the gas


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molecules and frameworks. The uptake capacity to hydrocarbons increase with the increasing of the gas polarizability (CH4 = 25 × 10-25, C2H6 = 44 × 10-25 and C3H8 = 63 × 10-25 cm3). The lowest value of Qst for CH4, which is in comparison with that of C2H6 and C3H8, indicates that the interaction between CH4 and the adsorbent is weakest.

Fig. 5 CO2, CH4, C2H6 and C3H8 adsorption isotherms at 298 K along with the dual-site Langmuir Freundlich (DSLF) fits (a, c); Gas mixture adsorption selectivity are predicted by IAST at 298 K and 100 kPa for JLU-Liu47 (b, d). With regard to the application of CO2 adsorption, the high uptake is an essential factor. Besides, the selectivity is another necessary determinant for the adsorbent since CO2 always mixed with other gases in practical condition. Because the kinetic diameters of CO2 and N2 are too similar to separate, it is crucial to construct porous MOFs materials with high CO2/N2 selectivity. We calculated the selective separation properties of CO2/N2 for JLU-Liu46 and JLU-Liu47. The selectivity of CO2/N2 (50% and 50%, 15% and 85%, 10% and 90%) is 39, 42 and 31 for JLU-Liu46 (Fig.6a, b) and 42, 45 and 36 for JLU-Liu47 (Fig.6c, d), respectively.


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The CO2/N2 selectivity of JLU-Liu46 and JLU-Liu47 are higher than those of BUT-11(31.5),51 BUT-10 (18.1),51 UiO-67 (9.4),51 NU-1000 (9)52 and NbO-Pd-1 (33.5).53

Fig. 6 CO2 and N2 adsorption isotherms at 298 K along with the dual-site Langmuir Freundlich (DSLF) fits for JLU-Liu46 (a) and JLU-Liu47 (c); Gas mixture adsorption selectivity are predicted by IAST at 298 K and 100 kPa for JLU-Liu46 (b) and JLU-Liu47 (d). The gas molecules selectivity of porous materials determines the performance of industrial fixed bed adsorbents. For a proper evaluation of the separation potential of JLU-Liu46 and JLU-Liu47, we test the performances of transient breakthrough using simulation methodology.54, 55

The following parameter values were used: length of packed bed, L = 0.3 m; voidage of

packed bed, ε = 0.4; superficial gas velocity at inlet, u = 0.04 m/s.56, 57 We investigate the results of breakthrough of equimolar 4-component CH4/CO2/C2H6/C3H8 mixture in a fixed bed packed with JLU-Liu46 and JLU-Liu47 operated at pressure of 100 kPa and 298 K (Fig. 7). The simulation results are represented in terms of a dimensionless time, τ, defined by dividing the


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actual time, t, by the characteristic time, Lε/u. The y-axis stands for the dimensionless concentration of the components at outlet of the fixed bed adsorbent. As for both JLU-Liu46 and JLU-Liu47, the breakthrough of C3H8, C2H6 and CO2 occur much later than CH4 which indicates the stronger binding of C3H8, C2H6 and CO2 as compared to CH4. So the selection of adsorbents should be primarily based on CO2/CH4, C2H6/CH4 and C3H8/CH4 separation performance. Although both JLU-Liu46 and JLU-Liu47 have separation performance, there are definite distinctions in occur time of adsorbate. Therefore, combining the theoretical IAST and breakthrough simulation verified that JLU-Liu46 and JLU-Liu47 are promising platforms for selective adsorption and separation of C3H8 from the mixture of CO2/CH4/C2H6/C3H8 hydrocarbons.

Fig. 7 Simulations of transient breakthrough characteristics for equimolar 4-component CO2/CH4/C2H6/C3H8 mixture in JLU-Liu46 (a) and JLU-Liu47 (b) at 100 kPa and 298 K. Grand Canonical Monte Carlo Simulation The density distribution of center-of-mass of CO2 molecules in JLU-Liu46 and JLU-Liu47 structures were simulated by Grand Canonical Monte Carlo (GCMC) method at 298 K and 1 bar.58 As shown in Fig. 8, it reveals that CO2 molecules in JLU-Liu46 and JLU-Liu47 are


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mainly adhere to the cages, which may have strong overlapping potentials. As for the whole structure, the CO2 molecules are apt to locate in both the open Cu metal sites and -NHCONHgroups (Fig. S17). Moreover, the density of CO2 molecules in JLU-Liu47 (Fig. 8b) is apparently higher than that in JLU-Liu46 (Fig. 8a). These results verify that the design and construction of MOFs frameworks by using polyhedron cages with more OMSs and LBSs sites would significantly enhance the CO2 adsorption.

Fig. 8 Density distribution of CO2 molecules center-of-mass of JLU-Liu46 (a) and JLU-Liu47 (b) at 298 K and 1 bar simulated by GCMC simulation. CONCLUSIONS In summary, two analogous PMOFs (JLU-Liu46 and JLU-Liu47) have been designed and synthesized by utilizing the SBBs strategy. JLU-Liu47 exhibits higher surface area and pore volume than JLU-Liu46 via altering the axial ligand. Benefitting from the introducing of classical Cu-paddlewheel and urea group, which provide high density of OMSs and LBSs, the two PMOFs approach high performance for CO2 capture. Meanwhile, both of the two PMOFs materials display prominent adsorption selectivity for CO2/CH4 and C3H8/CH4. It is also should


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be point out that the synthesis strategy adopted in this work is expected to be broadly employed in the fields of constructing MOFs materials with significant CO2 capture and gas selectivity performance for the efficient and practical applications. ASSOCIATED CONTENT Supporting Information. PXRD, TGA, additional structural figures, gas sorption data, crystallographic data. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * (Y. L.) Email: [email protected]. Notes The authors declare no competing financial interests. ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (Nos. 21373095 and 21621001) and the 111 Project (B17020). REFERENCES (1) Shekhah, O.; Belmabkhout, Y.; Chen, Z.; Guillerm,V.; Cairns, A.; Adil K.; Eddaoudi, M. Made-to-order Metal-organic Frameworks for Trace Carbon Dioxide Removal and Air Capture. Nature commun. 2014, 5, 4288. (2) McDonald, T. M.; Mason, J. A.; Kong, X.; Bloch, E. D.; Gygi, D.; Dani, A.; Crocellà, V.; Giordanino, F.; Odoh, S. O.; Drisdell, W. S.; Vlaisavljevich, B.; Dzubak, A. L.; Poloni, R.;


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Two analogous polyhedron-based MOFs with high Lewis basic sites

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